1. Technical Field
The present disclosure relates to vehicle control systems that enhance vehicle stability and performance.
2. Background Art
Stability-control systems are increasingly being used in automotive vehicles. In some prior two driven axle systems, a mechanical coupling is provided between the front and rear axles of the vehicle. In the event that one or both of the tires associated with the primary driven axle lose traction, the coupling apparatus, which is normally uncoupled, is commanded to couple the two axles so that torque is redistributed between a primary axle and a secondary axle. Although such a mechanical system provides improved performance compared to a purely braking approach such as with anti-lock braking systems, a mechanical system has several disadvantages. There is a delay between the time that the traction loss is detected and the mechanical coupler actually redistributes torque from the slipping tires of the primary axle to the wheels of the secondary. In situations such as encountering a patch of ice, in which road surface conditions can change very rapidly, a mechanical system is incapable of effecting a change in torque distribution sufficiently fast. Furthermore, due to frictional losses and torque transfer capability through the mechanical coupler, the sum of the torques supplied to the two axles is somewhat less than what the powertrain supplies to the primary axle. Thus, when the mechanical coupler is invoked, there is a drop in longitudinal performance of the vehicle, which may be particularly noticeable during acceleration. The ability of a mechanical system to redistribute torque may be limited in torque and further hampered by environmental influences, such as temperature.
A shift in torque distribution is also found to be useful in understeer and oversteer cornering situations. A mechanical coupler can be used to couple the primary and secondary axles to redistribute the torque between the axles such that the unexpected vehicle understeer or oversteer tendency is compensated. However, as with the desire to redistribute torque for a forward traveling vehicle encountering insufficient or excessive turning moment, the mechanical coupler reacts more slowly than desired and the torque redistribution is less accurately controlled in the coupler such that the operator of the vehicle notices a uncomfortable change in vehicle turning property.